a) Field of the Invention
The present invention relates to a miniature motor and, more particularly, to a structure of a balance adjusting mechanism of the rotor casing of the miniature motor.
b) Background Art
From the central portion of a motor frame 27, as shown in the prior art representation in FIG. 8, a shaft 21 is integrally projected upward. This shaft 21 is arranged on the frame with the inner races of a pair of ball bearings 25A and 25B which are spaced from each other at an axially constant distance. On the outer races of the paired ball bearings 25A and 25B, there is mounted a generally cup-shaped rotor casing 22 which is rotatably borne by the outer races.
On the outer circumference (i.e., the portion belonging to the motor frame 27, located at the lower side of FIG. 8) of the shaft 21, a stator core 24 is mounted. This stator core 24 is made by laminating a plurality of magnetic core elements and has a plurality of radially projected poles, which are individually wound with coils. These coils face a drive magnet 23. When the coils are energized, the drive magnet 23 is rotated together with the rotor casing 22.
The miniature motor thus constructed is subjected to a balance adjustment as a supplementary manufacturing step to reduce the rotational unbalance of the rotor casing 22. This unbalance is the result of an unevenness in the mass distribution around the center of rotation of the rotating parts (e.g., the rotor casing 22) of the motor which could be caused by the working error of the rotor casing 22 or the driving magnet 23 itself and by the assembly error which may be introduced when the drive magnet 23 is assembled with the rotor casing 22. With this unbalance, the motor is deleteriously affected by undesirable phenomena such as vibrations during rotation.
The aforementioned reduction in unbalance, which is made to create uniformity in the mass distribution of the rotating parts (e.g., the rotor casing 22) of the motor, requires that the resultant force and moment of the centrifugal forces of the rotating parts have to be reduced to zero. In a practical balance adjustment, an unbalance position is located from a rotating motor by means of a meter called the "balancer". Then, in the unbalance position of the rotor casing 22, a portion having a low mass distribution is made heaver by adding a balance correcting weight or adhesive 28 thereto, or a portion having a high mass distribution is made lighter by forming a hole 26 therein.
The method of correcting the unbalance by forming the hole 26 in the rotor casing 22 is disclosed in Japanese Utility Model Laid-Open No. 153769/1988. In Japanese Utility Model Laid Open No. 105480/1988, on the other hand, there is disclosed a method of restoring the balance by forcing a screw into the hole which is formed in the projection of the rotor casing.
In the prior art, however, the following problems arise if the hole is bored as a supplementary manufacturing step to effect the balance adjustment:
1. An additional step is required to remove the chips which are cut away by the boring question;
2. Facilities specially adapted for the boring operation are required;
3. The treated or plated surface of the worked portion is damaged by the boring operation; and
4. The correction after the boring operation deteriorates.
In contrast, the following problems arise if the correcting weight or the like is added to effect the balance adjustment:
1. Additional space has to be retained and provided for adding the correcting weight;
2. The balance adjustment is difficult because a quantitative weight addition is impossible; and
3. The adhesive or the like, if used, requires a drying time and, accordingly, an additional manufacturing step.